Time division multiple access or TDMA is a technique that allows multiple communication channels (e.g. telephone calls) to share the same frequency channel by dividing the signal on the channel into a plurality of dedicated time slots. Each user of the frequency channel is assigned one of the time slots in the frequency channel. The users transmit their data in discrete blocks within their designated time slots, each user having a particular time slot dedicated to that user. For instance, in a TDMA system that can support six communication channels (e.g. telephone calls) per frequency channel, a super-frame of, for example, 180 milliseconds may be defined. Each voice channel is assigned a time slot of one-sixth, or 30 milliseconds, of the super-frame period within which to transmit its data. Each call is assigned a different 30 millisecond segment of the 180 millisecond super-frame. In communications that demand a continuous flow of data such as voice communication, the data may be compressed, e.g., 180 milliseconds of voice data is compressed into each 30 millisecond time slot, so that the listener at the receiver end of the communication channel perceives a continuous stream of voice data, even though the actual transmission data comprises a stream of 30 millisecond long pieces of data separated by 150 millisecond intervals. A voice call spurt comprises a plurality of consecutive super-frames in which that voice call uses one of the 30 millisecond time slots in each super-frame to insert its data.
FIG. 1 illustrates an exemplary frequency channel in a TDMA communication system. The frequency channel 100 comprises a continuous stream of super-frames 1031, 1032, 1033, . . . , 103n. Each frame comprises a plurality (in this case six) 30 millisecond frames 1051, 1052, 1053, 1054, 1055, and 1056. Assuming for purposes of example that the super-frame is fully utilized, i.e., there are six active communication links A, B, C, D, E, and F, then each frame contains data from one of those communication links A-F, as shown.
As can be seen, the first super-frame 1031 contains a first block or portion of data from each of the six communication channels, A1, B1, C1, D1, E1, and F1. The next super-frame 1032 contains a different portion of the communication data for each of the communication links, A2, B2, C2, D2, E2, and F2.
The stream of super-frames continues essentially uninterrupted. When one of the links is terminated, the corresponding frame will become null or unused within the super-frame. When another communication link needs to be established, e.g., communication link G, then that unused frame may be used for that particular, new communication link.
TDMA is often used in wireless communication systems such as cellular telephone communication systems as well as land mobile radio (LMR) communication systems, including, for instance, industry standard P-25 Phase II and Harris' OpenSky™ system. The OpenSky™ system, for instance, achieves the equivalent of one voice call per 6.25 kHz of bandwidth using a combination of frequency division multiple access (FDMA) and TDMA. Specifically, each frequency channel is approximately 25 kHz wide and four voice channels are time division multiplexed within each frequency band.
LMR systems like the Harris' OpenSky™ system are commonly used by police, firefighters, municipal emergency squads, and the like. They often are implemented as rebroadcast type radio communication systems. Particularly, in typical cellular telephone communication systems, each user communicates on an essentially private channel with one other user. In rebroadcast type communication systems, on the other hand, some or all of the uplink channels to a base station are immediately rebroadcast from the base station to all the other radios in the system. More particularly, the base station receives a communication on an uplink channel and then rebroadcasts it on a downlink channel over a transmitting antenna for all other receivers within range to receive the communication.
Essentially, all electronic communication systems, and particularly wireless communication systems, have to deal with the possibility that data transmitted by a transmitter may not be received entirely accurately by the receiver. There are any number of possible sources of noise and interference, particularly in wireless communication systems, that can cause inaccurate reception of data, including multi-path interference, interchannel interference, ISI (intersymbol interference), Doppler shift, etc. Accordingly, communication systems often insert a substantial amount of overhead data in a communication channel in addition to the actual payload data that it is designed to communicate from one location to another (e.g. voice). For instance, control data and timing data often are transmitted within the payload channel or on a different channel. Furthermore, it is common to insert one or more parity bits (sometimes referred to as forward error correction bits), either per super-frame or per individual frame, in a TDMA system. As is well known in the industry, the value of the parity bit or bits is a function of the payload data according to a known algorithm. If the receiver determines that the parity bit values do not correspond exactly to the payload data it has received according to the algorithm, then it knows that some of the data likely has been received inaccurately.
In its simplest form, a single parity bit might be inserted indicating whether the number of ones (or zeros) in the payload data is an even number or an odd number. However, typically systems are much more complicated, including a plurality of parity bits, sometimes as many or more than the actual bits of payload data to which it corresponds, which can provide very detailed information indicative of which specific bits of the payload data were likely received incorrectly. For instance, a process known as forward error correction (FEC) is commonly used at the receiver to determine which bits were received incorrectly and to correct them.
In conventional TDMA communication systems, when there are fewer active communication links than the capacity of the system, time slots within the super-frame may go unused or may be used to transmit control data. For instance, in a TDMA system utilizing a six slot super-frame, if there are only four active calls during a particular period, only four slots will be used for data communication, and the other two slots in the super-frame will remain unused or may be used for transmitting control data or general broadcast messages.
In TDMA communication systems, each time a super-frame combining data from multiple communication channels is constructed, it may introduce latency into the individual communication channels. Usually, when a super-frame is assembled, it introduces a delay of at least the length of the frame. One super-frame delay period in a typical TDMA system is usually undetectable to the human ear. However, in a typical TDMA system, a frame of data of a communication channel may be assembled into, disassembled from, and reassembled into TDMA super-frames a plurality of times between the initial transmitter and the ultimate destination receiver.
For non-voice data communications, latency generally is of lesser concern because it usually is not significant if portions of data on a communication channel are delayed a few hundred milliseconds since the assembly of the data at the receiver usually is not particularly time sensitive. However, in voice communications, latency usually is more of a concern. For instance, latency of a quarter of a second in each of the uplink and downlink channels will add a half second of delay between the two human speakers in a two-way voice call, which can significantly interrupt the natural flow of human conversation.